Related applications have been filed under Ser. No. 779,137 filed Mar. 18, 1977 and Ser. No. 795,622 filed May 10, 1977.
The evaporation losses emanating from the liquids stored in the tanks, and due mainly to filling, emptying and "breathing" of these tanks, are substantially suppressed by the devices referred to in the above related applications.
These devices utilize collapsible containers, made out of suitable polymeric materials, such as polypropylene, polyurethane, fluoropolymers, etc. The side walls of these collapsible containers, built out of single or multilayer sheets or films of the thermoplastics just mentioned, are corrugated or folded to facilitate the collapsing or expanding of same.
The corrugations or folds are built into the container's wall prior to their installation in the storage tank they are to serve.
The construction of the corrugated surface, starting from a planar surface, requires special manufacturing methods.
One of such methods is thermoforming, which consists in the shaping of a preheated thermoplastic flat sheet in molds, shaped with the desired corrugations, by the application of pressure.
By this means, corrugated panels having the desired curvature to fit the circumferential tank's shell are obtained.
The size of a corrugated panel is limited to the largest size mold that is practical to fabricate and operate.
When the storage tanks sizes are considered, with sizes ranging from 25 feet to 200 feet in diameter and heights between 20 and 80 feet, it may be realized that many such corrugated panels are required for a tank.
The panels are subsequently joined together to form a curtain by welding their edges to a maximum size dictated by shipping, handling and mounting inside the tank considerations.
It has to be noted, that whereas a flat sheet is extremely flexible, foldable in any direction, and can easily be rolled into a relatively small diameter cylinder, the corrugated panels have built-in considerable stiffness which precludes them to be rolled and folded in any direction without creating undue stresses, except in the axial direction for the contraction or expansion of the corrugations.
It may now be realized that this fact imposes restrictions on the largest size for corrugated curtains that can be handled in practice.
This further requires that the individual curtains be joined together inside the tank, by welding their edges together in order to build the entire walls for the collapsible containers.
Other manufacturing method consists in the extrusion of corrugated rectilinear strips, followed by bending the reheated strips into the required curvature. These bent corrugated strips are joined into curtains by welding their edges together.
The related Gunther applications before mentioned are intended for use in liquid holding storage tanks.
By utilizing their same broad principles they are also applicable for gas holding storage tanks operating at substantially atmospheric pressure, and more specifically for the dry-seal type gasholders.
The space, which is occupied by the liquid and its vapors in the related applications, will be occupied by the gas to be stored, when these inventions are intended to be used as gasholding devices.
Dry-seal gasholders of the prior art consist of a tank with a vertically moving piston connected to the side wall by a series of parallel sealing rings of a special flexible synthetic rubber impregnated fabric chemically inert to the gas being stored. Since the piston freely floats on the gas, means of guiding and leveling this piston must be used.
Maintenance and leakage through the fabric seals, which wear due to the rubbing against the metalic surfaces they engage, are drawbacks that are overcome by the use of the present invention.